You might be able to rehydrate them. I'd leave them outside on a rainy night or two and grease them up.
HuH? Grease? Yup.
I read a book a while back about making English style walking sticks. Most had horn or antler handles. The old English saying about horn or antler was that it should be oiled "Once a day for a week, once a week for a month, once a month for a year and once a year for life."
I've done it with some sheds I picked up fresh and it's really kept them nice. Something like Mink Oil or Snow Seal works fine or maybe even just lard would work.
As for sounding different or deer telling the difference between old dry antlers and new ones, I'm not sure but I CAN tell you that the size of the antlers makes a big difference. Bigger antlers have a much deeper clunky sound than smaller "tinny" sounding ones. Unless they are just totally dead sounding, I doubt I'd worry about it. Most rattle bags, artificial antlers and such don't sound real to my ears but they work for the deer.